As a structure of a conventional crystal resonator, a crystal resonator 100 has been known in which a pair of lid bodies 92, 93 are bonded through bonding films 94, 95, 96 made of metal material, for instance, onto top and bottom surfaces of a crystal resonator plate 91 having a crystal resonator piece and a frame integrally formed therein as illustrated in FIG. 1A, for instance. External terminals 98 are formed on corner parts of the lower lid body 93 under the crystal resonator plate 91 so as to extend from side surfaces to a bottom surface of the lower lid body 93. The external terminals 98 provide electrical connection between electrode pads on a circuit board on which the crystal resonator 100 is mounted and the bonding films 95, 96 on the bottom surface of the crystal resonator plate 91. The bonding film 96 on the bottom surface of the crystal resonator plate 91 and the bonding film 94 on the top surface of the crystal resonator plate 91 are connected by through vias not illustrated. When a voltage is applied to the electrode pads on the circuit board, occurrence of a voltage difference between the bonding film 94 and the bonding film 95 causes resonance of the crystal resonator piece formed in the crystal resonator plate 91, so that a predetermined frequency is generated.
For mounting of electronic components on a circuit board, commonly, solder paste is applied onto a plurality of electrode pads on the circuit board, the plurality of electronic components are placed on the solder paste so that electrodes of the electronic components come into contact with the solder paste, the solder paste is simultaneously melted by reflow of the entire circuit board, and the plurality of electronic components are collectively fixed to the circuit board.
On condition that the electronic components are mounted on only one surface of the circuit board, problems hardly occur because the electronic components are collectively mounted by the reflow at one time. On condition that the electronic components are mounted on both surfaces of the circuit board, however, problems may occur because the reflow is performed twice due to demands on production.
FIG. 1B illustrates a problem that may occur when the crystal resonator 100 is mounted on a circuit board 80.
For mounting of the crystal resonator 100 on the circuit board 80, commonly, thin solder 82 is applied onto electrode pads 81 on the circuit board 80, the crystal resonator 100 is placed so that the external terminals 98 of the crystal resonator 100 come into contact with the solder 82, and the entire circuit board 80 is subjected to reflowing. Then the solder 82 is melted and soaks onto entire surfaces of the external terminals 98 of the crystal resonator 100. Then an Au constituent of the external terminals 98 is diffused into the solder 82 and an Au alloy is thereby formed. A melting point on the order of 200° C. of the Au alloy is lower than a melting point of 240° C. of lead-free solder and thus so-called solder erosion may occur in which the Au alloy is absorbed into the solder 82. When parts of the external terminals 98 undergo the solder erosion, the bonding films 95, 96 may be exposed on contact surfaces between the bonding films 95, 96 in the lower part of the crystal resonator 100 and the external terminals 98.
Upon performance of the second reflow in this state, the melted solder 82 may come into contact with the bonding films 95, 96 in the lower part. On this occasion, an Al constituent of the bonding films 95, 96 in the lower part may be diffused into the solder 82 and an Al alloy is thereby formed. A melting point on the order of 200° C. of the Al alloy is also lower than the melting point of 240° C. of lead-free solder and thus solder erosion may occur in which the Al alloy is absorbed into the solder 82.
FIG. 1B illustrates a state in which the bonding film 95 on the bottom surface of the crystal resonator piece 91 is made to recede by the solder erosion at a site designated by “X” in FIG. 1B. In such a state, the crystal resonator piece of the crystal resonator plate 91 fails to vibrate because the electrical connection between the bonding film 95 and the electrode pads 81 is lost. Then overall circuit malfunctions because the crystal resonator 100 generates a reference clock for operation of the circuit.
The following are reference documents:
[Document 1] Japanese Laid-open Patent Publication No. 2011-176787 and
[Document 2] Japanese Laid-open Patent Publication No. 2000-286671.